Bulk RF (radio frequency) radiation absorbing material is employed in applications such as anechoic chambers. Typically in such applications, bulk resistive material, such as polystyrene foam loaded with carbon, is shaped into predetermined geometric forms to provide a desired bulk geometric resistive gradient. Such geometric forms include pyramids or wedges. While such bulk absorbers can generally provide absorption over a wide frequency range, e.g., 100 MHz to 100GHz, these absorbers nevertheless suffer a number of disadvantages.
One disadvantage arises when there is a need to mount the geometrically shaped absorber to point outward from a vertical surface, e.g. the wall of an anechoic chamber. In such cases, the conventional absorber may sag due to its own weight and a degradation of absorption performance may result. A second disadvantage arises from the practical limitation that the absorber needs to be fashioned into geometric shapes corresponding to simple linear functions to obtain economic yields. As a result, it is not possible to optimize the resistive gradient presented to the incident radiation in order to maximize absorption performance.
A third disadvantage arises from the difficulty in cooling geometrically shaped bulk RF absorbers due to the thermal insulating properties of the material of which the absorber is typically composed. As a result, there is a cooling capability limitation on the power levels that can be absorbed. Further disadvantages encountered in using geometrically shaped resistive material are angular dependence of the absorption performance and initial reflections experienced from the tips and rear of the geometric shapes. Additionally, where the absorber is applied in an anechoic chamber, the thickness of the absorber is based on the low frequency requirement of the chamber, e.g., six foot pyramidal absorbers are typically required to provide a -50dB reflection at 1 GHz. Such large pyramidal absorbers can appear as large flat surfaces when illuminated by high frequencies, e.g., at frequencies greater than 18GHz. Thus, pyramidal absorbers sized for good low frequency performance will degrade performance at high frequencies.
Thin, flat absorbers are known as an alternative to geometrically shaped bulk absorbing material. One example is the Salisbury screen which comprises a carbon or metal impregnated resistive sheet spaced one-quarter of a wavelength over a ground plane. Another example is the Jaumann sandwich absorber which consists of plural resistive sheets each having a different resistivity and spacing over a ground plane. Disadvantages generally suffered by such flat absorbers are limited absorption bandwidth and/or frequency selectivity.
A flat absorber structure is disclosed in U.S. Pat. No. 2,977,591. The patent discloses use of a non-conducting fibrous material, such as animal hair, in which conductive particles, such as graphite powder, are distributed and act to absorb energy. While some of the above described problems suffered by the geometrically shaped absorbers are likely overcome by the absorber material disclosed in the referenced patent, the use of discrete conductive particles as energy absorbing material may lead to electrical discontinuity that can detract from absorber performance.